Presently, optical fibers for use in generally widely available optical fiber communication systems have a structure in which the outer circumference of a single core is surrounded by a cladding, and optical signals are propagated through the inside of this core to transmit information. In these years, information volumes to be transmitted are dramatically increased in association with the widespread use of optical fiber communication systems.
In order to realize an increase in the transmission capacity of these optical fiber communication systems, it is known that a multi-core fiber is used in which the outer circumferences of a plurality of cores are surrounded by a single cladding, and a plurality of signals is transmitted on light propagated through each of the cores.
Non Patent Literature 1 below describes an example of a multi-core fiber. In the multi-core fiber described in Non Patent Literature below, a single core is disposed in the center of a cladding, and six cores are disposed at regular spacings so as to surround this single core. In other words, the cores are disposed in a 1-6 configuration. Moreover, such a multi-core fiber is known in which cores are disposed in a 1-6-12 configuration (see Non Patent Literature 2 described below). In the multi-core fibers described in these Non Patent Literatures, the cores are disposed on the lattice points of a triangular lattice in such a manner that the cores are closely packed and disposed.
Furthermore, a trench type multi-core fiber is known in which each of the cores is surrounded by a low refractive index portion called a trench portion and a core element is formed to include the core and the low refractive index portion. According to this trench type multi-core fiber, light propagated through the cores is strongly confined in the cores. Therefore, it is suppressed that light propagated through each of the cores is leaked to the outside of each of the core elements, and the crosstalk between the cores can be decreased (see Non Patent Literature 3 described below).    [Non Patent Literature 1] K. Takenaga et al., “Reduction of crosstalk by quasi-homogeneous solid multi-core fiber,” OFC 2010, OWK 7 (2010)    [Non Patent Literature 2] K. Imamura et al., “19-core multi core fiber to realize high density space division multiplexing transmission,” in Proc. IEEE Photon. Soc. Summer Topical Meeting 2012, TuC 4.3 (2012)    [Non Patent Literature 3] K. Takenaga et al., “Reduction of crosstalk by trench-assisted multi-core fiber,” OFC 2011, OWJ4 (2011)